GOLDENTONE: Model 1755 Slimline

I saw this one pop up recently and decided to take a punt on it (thanks Damian).  For the money, I figured it was worth a look even if it only ended up being a decent chassis and set of transformers.

Turns out that this amp is a testament to the quality work that Goldetone produced - despite what has clearly been a very hard life, she works just fine (apart from the heavily corroded reverb tank). 

image from the eBay listing

Cosmetically, it has had a bit of a rough life (perhaps an understatement).  Where to begin? The original combo was cut down to a head, tolex removed, it was painted yellow, then black, and at some point it was covered in 1970s or 80s woodgrain vinyl.  The head still feels solid, so there’s something to work with.

I didn’t think the original faceplate was on the amp, so I was pleasantly surprised when the silver paint started wiping away with isopropyl alcohol, revealing the original panel underneath.  It's still in good shape too - maybe the dodgy paint work protected it?  

The reverb tank, on the other hand, was absolutely covered in heavy corrosion- no chance of salvage.

Inside the chassis was a much nicer.  It looks to be all original, with no obvious signs of previous work or failed parts.  It was dusty as hell, but I’ll take dust over a butchered circuit any day.

GOLDENTONE - 1755 SLIMLINE

I believe this is a Goldentone Model 1755 Slimline, based on the schematics I've seen.  Circa 1965, serial number 3967.  

The 1755 is regarded as one of the classic Goldentones - after a bit of time playing it, I can understand why.

FEATURES

Two channels, both have bass and treble controls.  One channel has reverb and trem.  

40watts

The original open back cab would have had 2 x 12" alnico speakers.

PREAMPS

Miniwatt 12AX7s.  

Tone controls

James style, bass and treble

Reverb / trem channel

First gain stage

Ra 220k

Rk 3k3, 25u bypass

Second gain stage

Ra 220k

Rk 1k8, 25u bypass

Normal Channel

First gain stage

Ra 100k

Rk 3k3, 25u bypass

Second gain stage

Ra 100k

Rk 3k3, 25u bypass

All pots 1MC (Volume, bass and treble)

REVERB

Valve driven, Miniwatt 6GW8

Pioneer EAV-201 reverb tank fitted (very small, 2 spring)

Rola LRH3 transformer

Transformer bench measurements

10.4v, 440Hz sine wave applied to primary:  result, 3.3v on secondary

That gives a turns ratio of:  10.4 / 3.3 = 3.15:1

Because impedance ratio is the square of turns ratio:  3.15² ≈ 9.9:1

So the transformer reflects about 10 times the tank/input impedance back to the 6GW8 pentode.

As a replacement, I'm using an old reverb tank from a Peavy.  Part number 710-1111.  DC resistance measures:  input ~50 ohms, output ~250 ohms.  It's probably an Accutronics 4EB2C1B, 600ohm input impedance, which will reflect 6k to the 6GW8.  That’s a very reasonable load, and it sounds great.   

Reverb level is completely independent from the preamp volume control.  i.e. the channel volume can be all the way down, but you can still hear the reverb.  The reverb send is post tone controls - before the volume control (attached to pin 3 of the volume pot).   

Reverb can be turned off via a foot switch.

TREM

12AX7 oscillator, modulating the cathode of a preamp triode.  

No depth control, just speed.

The trem pot is also the power on / off switch for the amp.  The trem is turned on / off by a foot switch.

PHASE INVERTER

12AX7, longtail pair

100n coupling caps

22k negative feedback from OT secondaries

POWER AMP

2 x Hitachi 6DQ6-B fitted - although schematics show 6DQ6-A (6GW6)

47 ohm resistor to screen

10k grid stopper resistors

470k grid leak resistors

Fixed bias -30  Bias not adjustable - no pot

OUTPUT TRANSFORMER SPECS (OT)

A&R 2700

Primary 3k3

Secondaries 16, 8 & 4 ohms

Output transformer bench measurements

84 ohms DCR primary

Test conditions:

10.5v, 440Hz sine wave applied to primary:   result, 0.58v on secondary

Turns ratio 18.1:1  

Primary

2.6kΩ at 8 ohms

5.2kΩ at 16 ohms

This is different to the specs above - possibly the result of construction variations and a low-voltage test.

POWER SUPPLY

A&R 5892 (which is different to schematics)

Silicon bridge rectifier

Rola choke TV301, May 1965

B+ about 360v

My little moment of joy, finding the original faceplate under the silver paint.  They may be original knobs, but I'm not sure. 

The 6DQ6 valves have their anode on a top cap - hence the warnings. 

A&R 5892 power transformer &  a big Ducon filter cap for the power supply.

Choke & reverb transformer& 6GW8 valve for the reverb

Prior to dust removal - all the mustard caps look fine, no cracks.  Can't see any resistors that have been heat-stressed.

They used an interesting method to mount components on the preamp and phase inverter.  There's a thread through the valve base, with another valve base on the other end of it.  It kind of makes a little tree for components to be vertically mounted.  

It is kind of difficult to get to the valve socket end, so maybe not the most practical method when compared to the tag stip mounted components.

This was a reverb tank at some stage in life.  

This is after most of the horrific vinyl has been removed, showing the remnants of various paint jobs underneath.

SCHEMATIC, GOLDENTONE MODEL 1755

Compared to some schematics floating around, mine has a few differences.   Worth noting that there are some major changes between the 1755 and some later Reverbmasters.  

• Some preamp plate and cathode resistor values differ   
• Reverb send and return paths are different   
• 47Ω screen resistors, not 47kΩ as shown on the schematic
• 10k instead of 180ohm resistor used for screen power filtering
• 4n7 capacitor across the reverb level control
• different power transformer is used

The measured voltages in my amp are very close to the schematic values overall, although the screen voltages are slightly lower, possibly due to the larger resistor in the filter section.

I liked the look of this schematic, so I made a few rough and ready modifications to match the values in my amp.

GOLDENTONE CATALOG IMAGES

FURTHER READING

https://www.aggh.net/discussion/index.php?topic=54693.0

GOLDENTONE: Model 1776 Guitar Amplifier

This is a Goldentone 1776, which is essentially a Goldentone 1774 with trem.   It had been reshoused in a 2 x 10 cab with Oxford speakers (one ceramic, one alnico), with the amp chassis mounted on the bottom, with the control panel up the top.  The amp would have been mounted the same way in its original 1 x 12" cab.

Based on the fact that it's made by Rose Morris, this would have been made before 1965.  Serial number 1375.  The 1776 models are listed as being made in 1961 - 1962

Apart from a DIY cab, I think it has had a little bit of work done on it.  There appeared to be an extra resistor in the power section - the rest looks stock, and I'd say there's a good chance that these are the original valves.  

Like its sibling, this amp has volume and tone controls before the first triode - which is kind of pointless.  It's low impedance, and it only really sounds good turned all the way up.  It basically puts another set of volume and tone controls before the amp, which you already have on your guitar.

GOLDENTONE MODEL 1776 GUITAR AMPLIFIER

• Single-ended 10w valve amp

• Volume 250k, Tone 250k, Depth 250k, Speed 2meg

• Silicon rectifier - looks like a voltage doubler

• 1 x 12AX7 preamp

• NFB to the first triode

• 1 x 6AU6 pentode driven tre, with Bakelite trem foot switch

• 1 x EL34 for the output

• Ducon caps

• OT  A&R 2584.  2k5 @ 16ohm

• PT A&R type 1971

Trem oscillator

POWER SUPPLY

• A&R type 1971 power transformer.

• Silicon rectifier, voltage doubler. 

• B+ 285v

SINGLE-ENDED OUTPUT TRANSFORMER

• A&R Type 2584

• Turns ratio 12.4

• 16 Ω speaker reflects 2.5 kΩ to the primary

COUPLING CAPS

These wax coupling caps had to come out - they didn't measure well at all - they never do.  When I tested the amp, it was sounding weak and thin.  Caps have been replaced.

GOLDENTONE 1776 - DRAFT SCHEMATIC

I've roughed out the main amplifier stage, and I'm nearly finished with the tremolo circuit. 

TREMOLO CIRCUIT

The 1776 uses a small 6AU6 pentode for the tremolo - the circuit is similar to the Silvertone 1482. 

I had to replace all of the caps, as they were well out of spec and a little leaky.

MODS

Modding a vintage amp is a little controversial, but this one was barely working when I got it, and it can easily be changed back. 

• Volume and tone moved to after the first triode

• Leaky wax paper coupling caps were replaced with slightly larger values, as while the amp sounded great when cranked with the smaller caps, they didn't do it any favours running clean.

• The tremolo had to be rebuilt to get it working, and as I had changed the volume and tone controls, I had to move the point where it was inserted into amp.  The trem still isn't perfect, slight tick when the amp is cranked.

• A couple of filter caps were replaced.

ROSE MUSIC: CSA. Model 35 PA Amplifier (Goldentone).

This one’s a rare find – a Communication Systems of Australia valve PA head, ~35 watts from around 1967 / 68.  It's super clean, fully functional, with all parts within spec.

Anyone familiar with vintage Australian guitar amps will recognise the front panel straight away: it looks just like a Goldentone.  That’s no accident.  These were built by Goldentone’s parent company, Rose Music in Melbourne, and were sold under the CSA, Goldentone and Zephyr badges.

The layout is typical for the period, with mic and pickup (turntable) volume controls and a single tone control. Like a number of Australian amps of the era, the power section uses TV valves – a quirk that seems to be unique to local designs.

• A pair of 6CM5 in the power stage 

• 12AX7s for the preamp and phase inverter.

• A&R power transformer

• A&R type 2766 Output transformer, marked 100v, 70v and 50v.

• Silicon bridge rectifier - ITT MB4 

• Mic input is grid leak biased, low impedance.

• Cathodyne phase inverter, with NFB from transformer winding

• Additional input straight into the phase inverter so the amp can be used as a slave (octal plug)

• Simple treble roll-off tone control

• Pots 500k volume, 250k tone

COMMUNICATION SYSTEMS OF AUSTRALIA, MODEL 35

CSA 35 OUTPUT TRANSFORMER

A&R Type 2766 output transformer with a constant-voltage secondary (50/70/100 V taps). I ran basic DC resistance and low-voltage AC ratio checks to fingerprint the winding and estimate the primary load seen by the output valves (2× 6CM5).  Note that I have not included the negative feedback winding yet.

16 Ω on 50 – 70 is about 3.8 kΩ p-p reflected.   Datasheet spec is 3.5 kΩ for 2 × 6CM5 at ~300 V B+

Winding / Tap	DC Resistance (Ω)	Measured VAC	Turns Ratio
Primary (p-p)	85	14.49	—
COM – 50	7.1	2.355	6.15 : 1
COM – 70	9.7	3.265	4.44 : 1
COM – 100	14.5	4.816	3.01 : 1
50 – 70	—	0.900	16.1 : 1

There's a spare noval socket on the far right, and the octal behind it is used as an input for the phase inverter.  I have seen another of these that had 2 x Mic inputs, using the additional socket for another preamp.

SOLID STATE RECTIFIER

The black square is the solid-state bridge rectifier marked as MB4 ITT - nice and compact.  The network sitting next to it is the negative bias voltage supply.

Preamp and phase inverter:  Blue, yellow and white hookup wires are colour-coded for plate, grid and cathode.  Green for ground.

CSA 35 VALVE AMPLIFIER SCHEMATIC

C8 is unusual in that it seems to be on a separate winding, compared to the rest of the output transformer.  It may be some form of shield.  Looking at the phase inverter and negative feedback network, they seem to have gone to some effort to reduce high-frequency oscillations / interference.  R12 / C9 is another example.

Voltages

B+ 380

Screens 188

Plates 375

Bias -32.5

CSA 35 CONVERSION NOTES

Despite being a little reluctant to touch the beautiful work done by the person who built it - it needs to function as a guitar amp.   

First things first - at 100k, the input impedance was too low.  Swapped the 100k for a 1meg resistor to bring it up a bit.   An input jack was added.

While the tone control was OK, a simple low-pass filter is a bit boring, and there was a spare knob...   Goldentones often have a James tone controls for bass and treble (sometimes called passive Baxandall), so I installed one to keep to the theme.  I used mustard caps and a low-value tropical fish capacitor to be consistent with the parts used in the amp.  

A bypass cap was added to the gain stage of the cathodyne phase inverter to make up for the loss of gain introduced by the tone controls, and a grid leak resistor was added to where the old tone control was in the circuit.

It sounds great - it's a solid little amp.

FURTHER READING

Oz Valve Amps

Aussie Guitar Gear Heads

The Trainspotter's Guide to Goldentone Amplifiers  (page 58)

GOLDENTONE: Vintage Goldentone Guitar Amplifier, Model 1774

Picked this up today - it’s a Goldentone guitar amp from the early sixties.  Goldentones were made in Melbourne, Australia when imported amps were a rare and expensive sight due to import tariffs.

Like a few early Goldentones, this amp does not have a name or model number marked on the amp itself.  It's a 1774 model ~1960/1961 from the 1770 - 1780 series.  I also have a post about a 1776 here.

Thanks to Peter Field from the Goldentone facebook group for most of the information in this post - basically anything related to dates, the model and confirming speaker details. 

GOLDENTONE 1774 AMPLIFIER SPECS

• Blue canvas covering

• Volume and tone controls

• 2 x inputs, single channel

• 1 x Mullard 6CA7 (EL34)

• 1 x Ten 12AX7

• Silicon rectifier

• A&R transformers

• M.S.P. 12PS speaker 12” 16 ohms

• 10 watts

• Serial number 1189

I've not completely opened it up to look at how it's constructed, but one odd thing to note is that the volume and tone control is at the front end of the amp, before the 12AX7, and they appear to be configured the same way as a guitar volume and tone pot.

To the right of the volume and tone pots, additional holes for pots can be seen.  Looks like this model shared the same base plate as the tremolo model.  

M.S.P. 12PS speaker is dated September 1958.  Goldentone did use M.S.P. in addition to Rola speakers.  The 12PS was discontinued in 1960, the 12PQ is more commonly found in Goldentones - at least when Rola's are not installed.

A&R Transformers were made in Melbourne at the time and were common across most Goldentone amplifiers.

It has a fairly early serial number, as I've read that they started at 1000

It has a Mullard 6CA7 that was made in Holland, dated December 1959, in the power section and a Ten 12AX7 for the preamp 

ROSE MORRIS 61 - 62 CATALOG

GOLDENTONE 1774 SCHEMATIC

I haven't had this amp for a while, so this trace is based on the previous photos and the 1776 that I currently have, which is the same amp with trem.  There are a couple of very minor differences, such as 2k2 cathode resistors on one and 3k3 on the other.  But fundamentally, this is the amp.